Arenechromium tricarbonyl complexes

Diastereoselective lithiation of chiral arenechromium tricarbonyl complexes... 

Arenes are inert to nucleophilic attack and normally undergo electrophilic substitution. However, arenes coordinate to Cr(CO)6 to form the i/fi-arenechromium tricarbonyl complex 79, and facile nucleophilic attack on the arene generates the anionic jy5-cyclohexadienyl complex 80, from which substituted arene 81, or cyclohexadiene is obtained by oxidative decomplexation. In this reaction, strongly... 

In addition to alkenes, arenes can sometimes be used as radical acceptors in Sml2-mediated carbonyl-alkene couplings. For example, Schmalz reported extensive studies on ketyl additions to arenechromium tricarbonyl complexes 66,67 tetralin-Cr(CO)3 complex 49 underwent reductive carbonyl addition to the aromatic ring upon treatment with Sml2 to furnish the skeleton of the naturally occurring aryl glycoside pseudopterosin G (Scheme 5.37).66,67 Here, the bulky metal tricarbonyl group not only serves to control the... 

Anti-Bredt alkenes, 56, 137, 217 Antimony(V) fluoride, 20-21 Aphidicolin, 205, 206, 294 Aplysistatin, 512 Apotphines, 201, 513-514 Arachidonic acid, 242-243 a-Arenechromium tricarbonyl complexes, 21, 117-118... 

Addition of earbanions to ir-arenechromium tricarbonyl complexes. Experimental details for addition of earbanions to n -benzenechromium tricarbonyl and for subsequent transformations (oxidation, reaction with electrophiles) of the resulting addition compounds have been published. ... 

Substituent effects in addition of earbanions to 7r-arenechromium tricarbonyl complexes have been examined. Methyl and chloro substituents give mixtures of ortho- and mefa-substitution, with the amount of mem-isomer increasing with bulk of the anions. Methoxyl and dimethylamino substituents are strongly meta -directing. The more hindered 3-position of 1,2-dimethoxybenzene is substituted even by a tertiary carbanion. Naphthalene shows 99% a -substitution. A trimethylsilyi group is strongly para-directing. 

Complexed Aryllithiums. Arenechromium tricarbonyl complexes are inctiilatcd with n-butyllithium and TMEDA in THE at -78° to afford chromium Iriciirbonyl complexes of aryllithiums. Metalation oeeurs specifically ortho to... 

Transition-metal-stabilized carbocations can be generated from functionalized butadieneiron carbonyl or arenechromium tricarbonyl complexes [92], Reactions of such carbocations formed from chiral complexes have been studied, but low selectivities are usually observed [526, 528, 535]. However, chromium tricarbonyl complexes derived from ephedrine 5.66 suffer cyclization in acidic medium. After decomplexation, c/s-tetrahydroquinolines are formed with a high diastereo-and enantioselectivity [540,542] (Figure 5.44). 

The reactions of boronates 2.68 (Re = Rz = H, R = i-Pr) are somewhat less selective [698], However, by using the arenechromium tricarbonyl complex of benzaldehyde or dicobalt hexacarbonyl complexes of a-alkynylaldehydes, homoal-lyl alcohols are obtained with a high selectivity after decomplexation [722, 1203] (Figure 6.44). These selectivities are interpreted by distorted chair transition states (Figure 6.44). In the reactions of allylboranes, the approach of the aldehyde minimizes both the steric interactions with the boron substituents and the eclipsing 1,3-interactions of the aldehyde C-R bond with the B-C bond. In the case of boronates 2.68, repulsive interactions between the oxygen lone purs are also avoided [698,1204] (Figure 6.44). 

Among the other acrylates of chiral alcohols used in cycloadditions with cy-clopentadiene, reactions of the monobenzoate of 2,2,6,6-tetramethyl-3,5-hep-tanediol [1398] under TiCl4 catalysis are highly selective, as are reactions of dural arenechromium tricarbonyl complexes 9.61 mediated by stoichiometric amounts of ZnCl2 [548], In this last case, the R group must be bulky (R = 1-Np, 2,4,6 (Figure 9.30). 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

Nucleophilic aromatic substitution can also be promoted by jr-complex formation but chromium complexes are particularly useful for realizing this type of activation as palladium or nickel do not lead to stable JT-complexes with arenes. For example, Jt-arenechromium tricarbonyl complexes react easily with nucleophiles, even when electrodonating substituents are present on the aromatic ring. This reaction has been used in two steps for the synthesis of carenone B (a sesquiterpene), one of these steps being an mrm-molecular nucleophilic attack with formation of a spirobicyclic system [22]. 

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